In recent years, as the capacity of a magnetic disk drive has been getting higher, the recording density and track density have been getting higher. As the track density has been getting higher, the interval between adjacent tracks (i.e., the track pitch) on a disk (magnetic disk) has been getting narrower.
As the track pitch has been getting narrower, data magnetically written in tracks adjacent to a target track might be magnetically degraded when data is magnetically written to the target track with a head (magnetic head). That is, the magnetization of adjacent tracks might be reduced. The reason is that a magnetic field generated by the head includes a component that makes no contribution to data writing, that is, a leakage magnetic field.
The effect of the leakage magnetic field on adjacent tracks is accumulated each time data is written to the target track. Therefore, as the number of times data is written to the target track increases, the magnetization of adjacent tracks are reduced further. The phenomenon of a decrease in the magnetization (i.e., demagnetization) of adjacent tracks is called a side erase.
As the demagnetization of adjacent tracks (or side erase) progresses, the error rate in reading data from the adjacent tracks increases. Before long, even if an attempt is made to restore the data in the adjacent tracks making full use of an error correction code (ECC), it will be difficult to restore the data. As a result, the data will be lost.
To overcome this problem, a recent magnetic disk drive requires track refreshing (rewriting) for restoring the magnetization of adjacent tracks before the progress of the demagnetization of adjacent tracks makes it impossible to read data from the adjacent tracks. That is, a write count is monitored for each zone or for each track. When the write count has exceeded a threshold value, each track in the corresponding zone or the corresponding track (more specifically, a track adjacent to a track whose write count has exceeded the threshold value) is refreshed.
Track refreshing is known as the operation of reading data from a demagnetized track and writing the read data to the track again. Rewriting data to the track (i.e., track refreshing) causes the magnetization of the track to be restored.
The demagnetization of adjacent tracks (side erasing) also takes place due to position errors in positioning the head on a target track to which data is to be written. In recent years, to cope with this problem, one known magnetic disk drive has a write inhibit slice set in each sector on a disk. In such a magnetic disk drive, when data is written to a target sector on a target track, a shift of the head position from the center line of the target track is detected. When the detected shift of the head position has exceeded the write inhibit slice corresponding to the target sector, the target sector is inhibited from being written to. This can prevent the data in a track adjacent to the target track from being erased (or rewritten) due to the shift of the head position.
As described above, the recent magnetic disk drive performs track refreshing when the write count for each zone or each track has exceeded the threshold value. This makes it possible to prevent the data in a track from being unable to be read due to demagnetization. However, when track refreshing is performed frequently, the performance of the magnetic disk drive decreases. Therefore, the threshold value of the write count tends to be set relatively high.
Even if the write count, for example, the number of times a second track adjacent to a first track has been written to, is in a range not exceeding the threshold value, the error rate in reading the first track is liable to increase as the write count increases. When the threshold value is set relatively high, this tendency increases. As described above, when the error rate in reading a track has increased, the number of times the track is retried being read from increases. Then, it takes a long time to read data from the track; on the contrary, the performance of the magnetic disk drive might decrease.